It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated, when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These- have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K. More recently, an oxide containing lanthanum, barium, and copper was discovered which became superconductive at temperatures of about 30.degree. K., and in some circumstances, at temperatures of about 20 degrees higher. Current advances have identified materials which become superconductive at temperatures near 100.degree. K., such that liquid nitrogen cooling could be used. Of special interest are ceramic materials which have reduced electrical resistance properties that are stable over time such that they could be developed for use in practical applications. While the phenomena of reduced electrical resistance and even superconductivity has now been observed at liquid nitrogen temperatures or above, these properties are still considered to be achieved primarily at low temperatures when compared to ambient conditions. However, there is some indication that ceramic materials might be formulated which can reliably exhibit reduced electrical resistance and perhaps superconductivity at ambient conditions.
A composition having an approximate unit cell formula of Y.Ba.sub.2.Cu.sub.3.O.sub.z, where z is typically about 7, and various related materials, represents a particularly promising group of ceramics for superconducting applications. The compositions are typically formulated from precursors which can be mixed to provide the desired ceramic. In one formulation for these ceramic materials, for example, carbonate and/or oxide powders of the solid elements are mixed and raised to a temperature of about 1,000.degree. C., driving off volatile materials, such as carbon dioxide. The mixture is reground and reheated, ordinarily several times to improve the intimacy of the mixture, and then can be pelletized, sintered for several hours, and then gradually cooled to below 250.degree. C.
Pellets have proven convenient for research applications properly involving ceramic superconductive materials since they can be readily formed by pressing together the powdered materials and binding them by a sintering process. These ceramic materials are typically brittle such that they are also more readily handled in pellet form. However, commercial applications of superconductors are likely to require substantial quantities of such materials in useful shapes such as tubes, rods, wires, or sheets, and other techniques for conveniently and reliably shaping these ceramic materials while maintaining their ability to conduct electricity with reduced resistance are being sought.
Reportedly, one procedure has been developed in which the ceramic powder is encased in a thin tube of metal such as silver, and then drawing the filled tube to form a wire. Evaporative techniques have also been reportedly used to produce films of superconducting materials from multiphase material comprising yttrium, barium, copper, and oxygen. In still another procedure, the ceramic powder, or even its ingredients, are mixed into an organic binder such as polyethylene glycol which is then extruded to form a plastic wire. After the wire is formed into the desired shape, the binder is burnt off and the residual powders are sintered to form the product filament. Tapes have also been produced by embedding ceramic particles in organic material to produce a flexible tape which can be shaped and then sintered. The conductive performance of the final ceramic material is known to be dependent upon the uniformity of element distribution throughout the composition. A common objective in any of the techniques for formulating and processing superconductive materials is to assure intimate mixing of precursor materials to provide a relatively homogeneous ceramic produce.